Tonight in New Hampshire the two most viable candidates so far in the GOP race will face off. Mitt Romney is vulnerable in ways that threaten his nomination; Tim Pawlenty’s challenges are personal.

Mitt Romney, of course, is hobbled by RomneyCare, or what Pawlenty will tauntingly call “Obamneycare.” Tonally, he sounds like a corporate executive (which he was), not a populist leader, which is what the base is seeking. What impresses the mainstream media and non-Republicans — a technocratic persona, a penchant for government activism — is what turns off the people he has to woo to win the nomination (unless he can slip by while hard-core conservative voters divide up their votes among different contenders).

Meanwhile, Pawlenty is cultivating the base by going after both Obama and Romney, as we saw on his “Fox News Sunday” performance. Chris Wallace, frankly, gave him an awfully tough time over his ambitious growth goal (5 percent over 10 years) and the extent to which his tax cuts will, as Wallace put it, “blow a hole in the national deficit.” His answers might seem frustratingly vague to some, but to conservatives looking for a pro-growth agenda he was heading in the right direction. He has branded himself as the pro-growth candidate, refusing to focus exclusively (as some candidates have) on the spending side.

His retort to Wallace’s budget skepticism (“So, is this aggressive and bold? Absolutely. But I don’t buy into the declinist view and attitude of President Obama that we’re going to settle for anemic growth or average growth or that America’s going to be laggard.”) is winning him fans on the right. And he could win over small-government conservatives, including Tea Partyers, if he is serious about taking government spending down to 18 percent of GDP, means-testing Social Security, block-granting Medicaid and proposing a Ryan-like Medicare plan (“It will include choices that people will have, including staying in the current Medicare program. But we’ll begin to incentivize people’s health care choices and selections based on higher quality care and more efficient care . . .”). The more criticism he receives from the mainstream media, the more kudos he’ll get from GOP activists.

Pawlenty’s challenge, unlike Romney’s, is a personal one. Can he project toughness, strength of character and some political showmanship that voters want? It is the superficial things (voice quality, body language, humor) that win over those who don’t follow politics minute by minute. So far he hasn’t found his comfort zone, though he may in the course of debates.

If the two most plausible candidates have real obstacles to overcome, there certainly is room for more entrants. They’ll have flaws, too. This is why we have elections — so imperfect candidates can fight it out and let the voters decide among the available choices.